Modeling the effects of sea spray on surface wind stress under hurricane conditions

Renee Richardson, Florida State University, Earth, Ocean and Atmospheric Science, Tallahassee, FL, United States, Mark A Bourassa, Florida State Univ, Center for Ocean-Atmospheric Prediction Studies, Tallahassee, FL, United States and Steven L Morey, Florida Agricultural and Mechanical University, School of the Environment, Tallahassee, FL, United States
Theory suggests that the balance between the surface enthalpy flux and the surface momentum flux at the air-sea boundary is one of the major physical processes governing tropical cyclone (TC) intensification. The surface enthalpy exchange is dependent upon the surface momentum exchange, so better understanding the behavior of the momentum transfer, i.e. surface stress, under TC conditions is our first step. In-situ observations of momentum exchange at high winds indicate a leveling off of the drag coefficient, a parameterization of surface stress, at ~25 - 30 m/s, which disagrees with prior literature stating the drag coefficient varies linearly with wind speed. We show that such a linear model can be modified to include momentum transfer associated with sea spray and produce the observed behavior of the drag coefficient. Our new surface drag model includes a recently parameterized bag breakup mechanism, which is concluded to be the dominant sea spray generation mechanism at wind speeds >20m/s. This mechanism’s stress contribution is parameterized in two separate parts: 1) the air resistance to the bags, a unique feature of this generation mechanism 2) the momentum acquired during droplet production. Normally, surface stress is modeled as only the form drag of the surface waves. Here, the inclusion of the spray physics results in a reduction in the form drag, meaning the presence of spray in the TC surface layer reduces the momentum felt by the surface.

We will show the theory behind spray generation at high winds, including the contributions of bags and droplets to the total turbulent stress under such conditions. The results from our spray-modified air-sea coupling model are compared to other surface stress models to illustrate the importance of spray inclusion when trying to match what is observed, while retaining the physics that works well at lower wind speeds. Lastly, we demonstrate a surface drag parameterization based upon our spray-modified stress model for future numerical model usage.